Optical modulators are currently used in optical communication systems to convert electrical signals representing data or voice into modulated optical signals. Optical modulators are typically based on direct or external modulation. With direct modulation, the optical source is turned on and off at intervals. With external modulation, the optical source is operated continuously and its output light is modulated using an optical external modulator.
Optical external modulators are superior to direct modulation in many ways. For example, optical external modulators are suitable for many high-speed applications and do not typically affect the wavelengths carrying the data signal as much as direct modulation. Furthermore, optical external modulators are often based on electro-optic, magneto-optic, acousto-optic, and/or electric field absorption type effects, thus providing additional design flexibility.
One example of a particularly successful optical external modulator is a Mach-Zehnder optical modulator, which is illustrated schematically in FIG. 1. The Mach-Zehnder optical modulator 10 includes an optical waveguide 20 formed on an electro-optic substrate 30, which for exemplary purposes is lithium niobate (LiNbO3). The optical waveguide 20 includes a first Y-branch 22, a first interferometer arm 24, a second interferometer arm 26, and a second Y-branch 28. A traveling-wave electrode structure 40 is provided near/adjacent the optical waveguide 20. The exact position and design of the electrode structure 40 relative to the optical waveguide 20 is typically dependent on the crystal axis of the lithium niobate substrate 30. For example, when the lithium niobate substrate 30 is x-cut, as shown in FIG. 1, the electrode structure 40 is positioned such that the first interferometer arm 24 is disposed between ground electrode 42 and hot electrode 46, while the second interferometer arm 26 is disposed between ground electrode 44 and hot electrode 46.
In operation, light is input into the modulator 10 from the left side and is output on the right. More specifically, the input light propagates through the optical waveguide 20 until it is split at the first Y-branch 22, where it then propagates equally along the two isolated paths corresponding to the two interferometer arms 24, 26. When a time varying voltage is applied to the traveling-wave electrode structure 40, an electric field is produced that propagates down the electrode structure 40, which is constructed to form a microwave waveguide. The electric field at least partially overlaps the two interferometer arms 24 and 26. In accordance with the electro-optic effect, the electric field causes the relative velocity of the light propagating through the two interferometer arms to change, thus creating a phase shift and producing constructive or destructive interference at the second Y-branch 28. The constructive and/or destructive interference produces an amplitude modulated optical signal, wherein the modulation corresponds to a modulated RF data signal used to produce the time varying voltage.
FIG. 2a illustrates a transfer function of the electrical to optical conversion for a typical Mach-Zehnder optical modulator, such as that shown in FIG. 1. The transfer function is a theoretically sinusoidal curve that represents the points at which the optical modulator will transition from no output to maximum output. For example, when the applied voltage is near a first value −Vπ/2 the modulator output is at a minimum. As the applied voltage approaches 0 the modulator output approaches 50% transmission. When the applied voltage is near a second value Vπ/2, the modulator output is at a maximum. The value Vπ is known as the “peak-to-peak” or “switching” voltage. The point A, which is approximately half way between the maximum and minimum peaks of the transfer function, is known as a quadrature point. To achieve maximum modulation efficiency, it is usually preferred that the time varying voltage (i.e., an AC type voltage) includes a maximum amplitude of Vπ and that the optical modulator be biased at quadrature (i.e., a DC bias voltage is set at the quadrature point). Accordingly, the time varying AC voltage is continuously swung around the DC bias voltage in a balanced fashion.
Optical external modulators, such as the Mach-Zehnder optical modulator discussed with respect to FIG. 1, have been found useful in both analog systems, such as cable television and/or radar networks, and digital systems, such as today's long-haul-terrestrial and submarine optical networks. When used in analog systems, the applied voltage is usually between −Vπ and Vπ, but does not typically reach the extremes of this range. When used in digital systems, such as with a traditional two-level digital data signal, the applied voltage is swung between 0 and Vπ and/or 0 and −Vπ, so as to generate the digital 1's and 0's in the optical domain. As a result, optical analog external modulators and optical digital external modulators are generally associated with different performance concerns, and thus designs.
In optical analog external modulators, the primary performance concern appears to be the lack of linearity in the transfer function. One solution to this problem is to cascade two optical modulators in series. For example, see U.S. Pat. No. 5,168,534 to McBrien et al., U.S. Pat. No. 5,148,503 to Skeie, U.S. Pat. No. 5,249,243 to Skeie, U.S. Pat. No. 6,091,864 to Hofmeister, and U.S. Pat. No. 6,535,320 to Burns, all hereby incorporated by reference.
In optical digital external modulators, the primary performance concern appears to be the high drive power required to switch between 0 and Vπ and/or 0 and −Vπ (i.e., the high driving voltage). Various attempts to lower the drive power of optical digital external modulators have been proposed. For example, in U.S. Pat. No. 6,304,685, Burns teaches etching the lithium niobate substrate, in U.S. Pat. No. 6,341,184, Ho et al. teach including a resonator near one of the arms of a Mach-Zehnder interferometer, and in U.S. Pat. No. 6,647,158, Betts et al. teach using a specific combination of crystal axis orientation, waveguide structure, electrode structure, and biasing of a Mach-Zehnder optical modulator to lower the required drive voltage.
A second performance concern of optical digital external modulators is the breadth of the modulation bandwidth. Modulation bandwidth is typically limited by the fact that the RF signal travels more slowly through the electrodes than the optical signal travels through the optical waveguide. Prior art methods of correcting velocity mismatch have included varying the electrode width, gap and thickness and/or varying the choice and thickness of a buffer layer deposited on the substrate.
A third performance concern in optical digital external modulators is the quality and/or integrity of the optical digital signal after it has been transmitted by the optical modulator (i.e., this factor determines the distance separating the transmitter from the receiver in use). The integrity of a digital optical signal is often characterized by an eye diagram, where a clear and symmetric eye diagram with well defined lines is associated with high transmission performance (e.g., minimal bit errors).
Referring to FIG. 2b, there is shown an eye diagram for an ideal, two-level digital signal. The eye-diagram is a superimposed plot of normalized amplitude versus time, for all the optical signals produced by the optical modulator. In other words, it shows where the digital 1's and 0's of all the bits overlap in one plot (the plot in FIG. 2b is actually two bit periods wide). The X's in the eye diagram are caused by the overlap of all the 1→0 and 0→1 transitions. The center of the X's determine the eye crossing level, which is shown having the ideal value of 50%. The digital signal is understood to be ideal in all properties except for finite (30 psec) rise and fall times.
Further discussion with respect to the quality of the digital signal and eye diagrams is provided in U.S. Pat. No. 6,687,451 to Sikora, hereby incorporated by reference.